Many of automotive bumper beams to be mounted on automotive bodies are formed by use of steel plate of 980 MPa·class so that a cross section orthogonal to the longitudinal direction has a B-shape. The “B-shape” referred to in the specification means a shape having a reference wall extending in a specific direction on the cross section and a pair of swelling parts each swelling from the reference wall in a direction orthogonal to the reference part to form a space inside thereof.
The principal roles of such an automotive bumper beam are (i) to absorb, in a collision with a collision object, the impact energy of the collision by itself through its own deformation and (ii) to cause, in the collision, side members on the lateral side of a vehicle body to absorb the energy of collision by transmitting the impact load to the side members to deform the side members. Namely, the bumper beam is configured to suppress the deformation of an automotive cabin as designed to protect the occupant from the impact by causing the side members to absorb the impact energy.
With respect to concrete structures of such a bumper beam, various technologies are conventionally known.
For example, Patent Literature 1 discloses a support for automotive bumper including a B-shaped sectional reinforcement having an upper cylindrical part and a lower cylindrical part, and a foam material filled in each of the cylindrical parts. This support attains reduction in weight thereof and prevention of buckling of the reinforcement by filling the foam material.
Patent Literature 2 discloses a bumper main beam having an upper wall, an upper-side intermediate piece, a lower wall, and a lower-side intermediate piece that constitute the B-shaped cross section, the upper wall and the lower intermediate piece being inclined downwardly, and the lower wall and the upper intermediate piece being inclined upwardly. In this structure, the upper wall and lower wall of the main beam are buckled in a bellows shape in a vehicular crash, whereby the safety to the occupant of the crashing vehicle and the occupant of the crashed vehicle is improved.
Patent Literature 3 discloses a back beam for automotive bumper, including two hollow members, a plurality of brackets mutually connecting the hollow members so that the hollow members constitute a B-shaped cross section, and a reinforcement member locally reinforcing a relatively weak center part that is the connection part between the hollow members.
Patent Literature 4 discloses a reinforcement for automotive bumper, including a web, a compressive flange and a tensile flange which constitute a hollow rectangular section. In this reinforcement, the overall bending strength is enhanced by setting the thickness of the web so as to be larger on the side close to the compressive flange with respect to a neutral axis than that of the web on the side closer to the tensile flange.
Patent Literature 5 discloses a bumper reinforcement having three ribs. The plate thickness of the intermediate rib of the above-mentioned ribs is set larger than those of the other ribs, whereby the deterioration of energy absorbing capability upon buckling of the three ribs is prevented.
Patent Literature 6 discloses a structural member having a pair of flanges disposed in parallel to each other. In this structural member, an FRP material is provided on the surface of the flange on the tensed side upon receipt of bending load (the flange disposed oppositely to the flange on the compressed side) of both the flanges, and the ratio of width to thickness of the flange on the compressed side is set to 12 or less. This enhances the energy absorbing quantity of the structural member.
Patent Literature 7 discloses a composite structural member including a steel pipe and a reinforcement pipe inserted thereto. The reinforcement pipe has an outer shape along the inner wall of the steel pipe, and a rib is formed on the inside of the reinforcement pipe. This ensures a sufficient strength.
Patent Literature 8 discloses a filled structure including a hollow member and a filler inserted thereto. The filler is excellent in energy absorbing performance, and is fixed to the inside of the hollow member. In this filled structure, satisfactory corrosion resistance is ensured.
Patent Literature 9 discloses a vehicle body structural member composed of a plurality of members differed in strength. In this vehicle body structural member, this difference in strength of each member causes a twisting moment in the vehicle body structural member to disperse the bending load to the other members. Therefore, the energy absorbing efficiency of the vehicle body structural member is improved.
Patent Literature 10 discloses a bumper beam provided with a bumper reinforcing material. The bumper reinforcing material has a hollow part, and a crush deformation preventing body is disposed in the hollow part. The crush deformation preventing body suppresses buckling deformation of the bumper reinforcing member to enhance the impact energy absorbing capability.
The conventional structural members described above have the following problems when used as a bumper beam.
An automotive bumper beam requires a high bending strength in a high-speed collision of a pole-like object with the front or rear of a vehicle body. Namely, the automotive bumper beam has to transmit, in the collision with the pole-like object, the load to side members without its serious deformation.
However, a steel automotive bumper beam has limitation in improvement in bending strength because of insufficient exercise of sectional performance. The causes of the insufficient exercise of sectional performance include (i) deflection of a flange (vertically extending wall part), and (ii) compressive buckling of a web (longitudinally extending wall part).
On the other hand, it is desirable to enhance the bending strength of the bumper without increasing the weight of the bumper as much as possible.
With respect to the above-mentioned issue of (ii), since the buckling load of the web is proportional to the Young's modulus and the cube of plate thickness of the material, the buckling of the web can be suppressed more efficiently by increasing the thickness of the material than by improving the strength of the material. However, the increase in thickness is associated with significant increase in overall weight of the bumper beam. Particularly, in a steel automotive bumper beam manufactured by roll forming, an increased thickness of the web results in an increased overall weight of the beam in proportion thereto since the overall thickness of the bumper beam is uniform. Thus, remarkable improvement in the durability performance (to buckling) per unit weight cannot be expected. Further, as the background, reduction in overall weight of a vehicle is recently required for reduction in CO2 emission.
Further, it is very difficult in manufacturing to attach a reinforcing material (addition product) within the automotive bumper beam, and the use of a large reinforcing material involves remarkable increase in weight and cost. On the other hand, the use of FRP (Fiber Reinforced Plastics) as a component of bumper beam for the purpose of reducing the weight offers little hope of preventing the compressive buckling of the beam.    [Patent Literature 1] Japanese Patent Application Laid-Open No. 11-334500    [Patent Literature 2] Japanese Patent Application Laid-Open No. 2006-218904    [Patent Literature 3] Japanese Patent Application Laid-Open No. 2005-8146    [Patent Literature 4] Japanese Patent Application Laid-Open No. 11-059296    [Patent Literature 5] Japanese Patent Application Laid-Open No. 2004-148915    [Patent Literature 6] Japanese Patent Application Laid-Open No. 2003-129611    [Patent Literature 7] Japanese Patent Application Laid-Open No. 2003-312404    [Patent Literature 8] Japanese Patent Application Laid-Open No. 2005-88651    [Patent Literature 9] Japanese Patent Application Laid-Open No. 2006-248336    [Patent Literature 10] Japanese Patent Application Laid-Open No. 2000-52897